1. Field of the Invention
The invention relates to a frictional hinge device used to pivotably support and hold a lid plate and a display of a portable business machine such as a laptop note type personal computer or the like at desired pivotal positions, and concerns to a portable business machine into which the frictional hinge device incorporated.
2. Description of Prior Art
In a conventional frictional hinge device incorporated into a laptop note type personal computer, frictional torque is used to hold a lid plate or a display of the personal computer at any pivotal positions. A support block is made of a synthetic resin and integrally molded with a metallic shaft. Due to a surface friction between the support block and the metallic shaft, the metallic shaft is held at the desired pivotal position while permitting the metallic shaft to pivot freely relative to the support block against the surface friction therebetween.
However, when an engagement relationship between the metallic shaft and the support block is not relevant, there arises no smaller variations on torque upon pivotally moving the metallic shaft relative to the support block. This induces abnormal noise due to a stickslip phenomenon when pivoting the metallic shaft relative to the support block, whereby decreasing the frictional torque to lose a good endurance due to an unacceptable amount of wear between the metallic shaft and the support block.
In order to overcome these inconveniences, a surface treatment (surface roughness e.t.c.,) against the metallic shaft member has been analyzed. The analyses has been directed only to the surface roughness of the metallic shaft in an axial direction, but not in a circumferential direction.
Almost no regard has been paid to a deviation from circular form and the surface roughness of the metallic shaft in the circumferential direction because cutting and buffing procedures are routinely implementing in machining revolution work pieces.
The inventors have been delving into the relationship between the metallic shaft and the support block, and as a consequence found it important to define a relevant relationship among the deviation from circular form and the surface roughness of the metallic shaft in both the circumferential and axial directions in order to improve the endurance capable of maintaining an appropriate torque for a long period of time.
Therefore, the present invention has been made with the above drawbacks in mind, it is a main object of the invention to provide a frictional hinge device which is capable of maintaining a stable frictional resistance between a shaft member and a support member for an extended period of time when holding the shaft member at desired pivotal positions upon opening and closing a lid plate or the like.
It is also an object of the invention to provide a portable business machine into which the frictional hinge device is incorporated.
With a support member made of a resin molded to surround a shaft member, the support member tightly engages with the shaft member due to a residual stress based on a shrinkage allowance after molding the support member in integral with the shaft member. During the molding process, the synthetic resin is injected into a die mold in which the shaft member was beforehand placed.
When a surface frictional resistance appeared between the support member and the shaft member is smaller than an exterior torque to which the shaft member is subjected, the shaft member pivotally moves relative to the support member. When the surface frictional resistance exceeds the exterior torque, the shaft member is held at a desired pivotal angle due to the surface frictional resistance.
With the support member and the shaft member molded integrally, the frictional hinge device is manufactured less costly.
An outer surface of the shaft member is cylindrically finished by means of extruding and cutting procedures. With the result of the inventors"" research and observation, when an innermost shift position (innermost dent V) against a true circle is more than 1.0 xcexcm, the inventors found that a torque holding rate abruptly falls so as to decline the endurance. This is true when a deviation xe2x80x9cP-Pxe2x80x9d from a circular form is more than 2.5 xcexcm. The deviation xe2x80x9cP-Pxe2x80x9d is defined by a difference between an outermost shift position and the innermost shift position with respect to a referential center circle of an outer surface of the shaft member.
With the innermost dent V and the deviation xe2x80x9cP-Pxe2x80x9d expressed as V less than 1.0 xcexcm and P-P less than 2.5 xcexcm, the shaft member is demanded to be treated so that the deviation from a circular form highly decreased. By finishing the shaft member to meet the above requirement through a quality control, a frictional hinge device superior in endurance is provided.
A surface roughness Ra of the shaft member decreases with the decrease of the deviation from a circular form. When there is no ruggedness on the finished surface of the shaft member, the support member tightly engages with the shaft member to coagulate at an engagement surface between the resin of the support member and the shaft member.
In order to avoid the coagulation against the resin, the shaft member is treated so that its surface roughness Ra is 0.05xcx9c0.20 xcexcm in the circumferential direction.
With this treatment, a smooth outer surface of the shaft member is insured while avoiding the coagulation against the resin. This insures an appropriate lubrication at the engagement surface between the support member and the shaft member, thereby preventing the abnormal noise from inducing when pivoting the shaft member relative to the support member.
With the shaft member having 0.15xcx9c0.30 xcexcm as the surface roughness Ra in the axial direction which is greater than the surface roughness Ra in the circumferential direction, a needed magnitude of friction is readily determined. This avoids the abnormal noise from inducing due to the slipstick when pivoting the shaft member relative to the support member while suppressing the wear between the shaft member and the support member, thereby repeatedly holding the shaft member at the desired pivotal positions for an extended period of time.
It is to be noted that various buffing treatments are used to the outer surface of the shaft member so that the surface roughness in the axial direction is greater than the surface roughness in the circumferential direction.
With the portable business machine into which the frictional hinge device is incorporated, a stable torque is maintained with the least amount of wear, which is capable of repeatedly holding a display-mounted lid plate member at the desired pivotal positions for an extended period of time.